Nanoparticles having metal non-oxide compositions (i.e., “semiconductor” or “quantum dot” nanoparticles) are increasingly being used in numerous emerging applications. Some of these applications include electronics (e.g., transistors and diode lasers), LED displays, photovoltaics (e.g., solar cells), and medical imaging. Quantum dot nanoparticles are also being investigated as powerful new computer processing elements (i.e., qubits). Semiconductor nanoparticles often possess a metal chalcogenide composition, such as CdSe and ZnS.
As a consequence of its small size, the electron band structure of a quantum dot differs significantly from that of the bulk material. In particular, significantly more of the atoms in the quantum dot are on or near the surface, in contrast to the bulk material in which most of the atoms are far enough removed from the surface so that a normal band structure predominates. Thus, the electronic and optical properties of a quantum dot are related to its size. In particular, photoluminescence is size dependent.
Several physical methods are known for synthesizing semiconductor nanoparticles. Some of the physical techniques include advanced epitaxial, ion implantation, and lithographic techniques. The physical techniques are generally useful for producing minute amounts of semiconductor nanoparticles with well-defined (i.e., tailor-made, and typically, uniform) morphological, electronic, magnetic, or photonic characteristics. The physical techniques are typically not useful for synthesizing semiconductor nanoparticles in commercially significant quantities (e.g., grams or kilograms).
Several chemical processes are also known for the production of semiconductor nanoparticles. Some of these methods include arrested precipitation in solution, synthesis in structured media, high temperature pyrolysis, and sonochemical methods. For example, cadmium selenide can be synthesized by arrested precipitation in solution by reacting dialkylcadmium (i.e., R2Cd) and trioctylphosphine selenide (TOPSe) precursors in a solvent at elevated temperatures, i.e.,R2Cd+TOPSe→CdSe+byproducts
High temperature pyrolysis of semiconductor nanoparticles generally entails preparing an aerosol containing a mixture of volatile cadmium and selenium precursors, and then subjecting the aerosol to high temperatures (e.g., by carrying through a furnace) in the presence of an inert gas. Under these conditions, the precursors react to form the semiconductor nanoparticles (e.g., CdSe) and byproducts.
Though the chemical processes described above are generally capable of producing semiconductor nanoparticles in more significant quantities, the processes are generally energy intensive (e.g., by generally requiring heating and a post-annealing step), and hence, costly. Accordingly, commercially significant amounts of the resulting nanoparticles tend to be prohibitively expensive. Furthermore, these processes tend to be significantly limited with respect to control of the physical (e.g., size, shape, and crystalline form) and electronic or photonic characteristics of the resulting nanoparticles.
The microbial synthesis of semiconductor nanoparticles is known. See, for example, P. R. Smith, et al., J. Chem. Soc., Faraday Trans., 94(9), 1235-1241 (1998) and C. T. Dameron, et al., Nature, 338: 596-7, (1989). However, there are significant obstacles that prevent such microbially-mediated methods from being commercially viable. For example, current microbial methods are generally limited to the production of semiconductor nanoparticles on a research scale, i.e., an amount sufficient for elucidation by analytical methods. In addition, current microbial processes generally produce semiconductor nanoparticles adhered to cell membranes. Accordingly, numerous separation and washing steps are generally needed.
Accordingly, there is a need in the art for a microbial method for the synthesis of semiconductor nanoparticles capable of producing semiconductor nanoparticles on a commercial (i.e., bulk) scale at a non-prohibitive cost. There is also a need for a microbial method of synthesis which provides substantially pure semiconductor nanoparticle product bereft of microbial matter, thereby reducing or eliminating separation and washing steps. There is also a particular need for such a microbial method of synthesis whereby characteristics of the nanoparticles (e.g., particle size, morphology, electronic or photonic characteristics, dopant composition, and doping level) are more precisely or uniformly controlled.